A method and a device for continuously crystallizing polyester granulate wherein granulate is passed through two fluidized beds connected in series is known in the art. The first fluidized bed is a bubbling bed and the second fluidized bed is a circulating fluidized bed.
An entering amorphous flow of granulate is heated and precrystallized in the bubbling bed with intensive movement and mixing with existing crystalline material. Heated air or an inert gas such as nitrogen is used for fluidization. The exhaust gas from the bubbling bed escapes upward and can be recirculated back to the bubbling bed. The flow of granulate emerging from the bubbling bed enters the circulating fluidized bed, which has a channel-like receptacle that is provided at the bottom with a perforated plate. The exhaust gas from the circulating fluidized bed can likewise be returned to the circulating fluidized bed in a circuit.
This method has the disadvantage that separate, individual circuits must be provided for the return and recovery of the process gas both for the fluidized bed and for the bubbling bed. This gives rise to increased installation effort, increased capital expenditures, and operating costs.
A method and an arrangement for producing low-hydrolysis polyester granulates in which a mixture of pellets and water produced by underwater granulation is fed into an agitating centrifuge as a pre-dryer as known in the art.
From the pre-dryer, pellets are conveyed via a classifying screen to a collecting vessel and then are subsequently conveyed to a delay silo. Air, which is delivered at approximately room temperature, flows through the delay silo. The exhaust air from the silo can be used to purge the collecting vessel and the pre-dryer. The air emerging from the delay silo is heated to a temperature of 140 degrees Celsius to 180 degrees Celsius using a heater. An injection condenser follows the pre-dryer in order to extract the water vapor from the exhaust air of the pre-dryer and to recover process water.
In this arrangement, the delay silo must of necessity be present and must be integrated into the arrangement in order to be able to use the exhaust air of the delay silo for purging the pre-dryer and collecting vessel. Consequently, the method is not suitable for applications in which no delay silo is used, or for applications in which a delay silo is operated at a different, distant location decoupled from the drying of the granulate. Moreover, the exhaust air of the pre-dryer, or of the injection condenser, is not used further and thus the heat energy contained therein remains unused and is wasted.
One object of the present invention is thus to overcome the abovementioned disadvantages and to specify a method and a device for drying and crystallizing water-flushed granulate that entail less installation effort and lower installation and operating costs.
Another object of the present invention is to specify a method and a device for drying and crystallizing water-flushed granulate that are versatile and more energy efficient in operation.
These and other objects of the present invention are attained by the present embodiments.
The present embodiments are detailed below with reference to the listed Figures.